Process for recovering heat from gas mixtures obtained by the thermal cracking of hydrocarbons

ABSTRACT

In a process for the production of acetylene and/or lower olefins from hydrocarbons the enthalpy of cracked gas is recovered in two stages by an oil wash. In the first stage of the oil wash using temperatures within the boiling range of the wash oil and approximately atmospheric pressures, a cycle of highboiling oil components is maintained and condensed lower boiling oil components which are recycled from the second wash stage are directly vaporized by the cracked gases. The saturated mixture of oil vapor and cracked gases formed in the first stage is fed to the second wash stage and condensed therein by indirect cooling.

United States Patent 1191 Voigt et al. 1 Apr. 3, 1973 s41 PROCESS FOR RECOVERING HEAT 3,283,026 11 [1966 Kandler et al. ...2( 0 m9 R FROM GAS MIXTURES OBTAINED BY 7/1972 Dorn ct a]. .208/48 THE'THERMAL CRACKING OF HYDROCARBONS Inventors: Hartman Voigt; Erich Schneck;

Klaus Giinther; Klaus Noha, all of Frankfurt/Main, Germany Farbwerke Hoechst Aktiengesellschaft vormals Meister Lucius Assignee:

8: Bruning, Frankfurt/Main, Germany Filed: Feb. 16, 1971 Appl. No.: 115,319

Int. Cl ..C07c 11/24 Field of Search ..260/679, 683; 208/48 0; 55/64 References Cited UNITED STATES PATENTS 2/1966 Otsuka et al ..260/679 R FOREIGN PATENTS OR APPLICATIONS 929,136 6/1963 Great Britain ..260/679 Primary ExaminerDelbert E. Gantz Assistant Examiner-J. M. Nelson Attorney-Curtis, Morris & Safford 57 ABSTRACT In a process for the production of acetylene and/or lower olefins from hydrocarbons the enthalpy of cracked gas is recovered in two stages by an oil wash. In the first stage of the oil wash using temperatures within the boiling range of the wash oil and approximately atmospheric pressures, a cycle of high-boiling oil components is maintained and condensed lower boiling oil components which are recycled from the second wash stage are directly vaporized by the cracked gases. The saturated mixture of oil vapor and cracked gases formed in the first stage is fed to the second wash stage and condensed therein by indirect cooling.

2 Claims, 1 Drawing Figure 1 PROCESS FOR RECOVERING HEAT FROM GAS MIXTURES OBTAINED BY TI-IETI-IERMAL CRACKING OF HYDROCARBONS The present invention relates to a process for recovering heat from a gas mixture obtained by the cracking of hydrocarbons or mixtures of hydrocarbons directed to the production'of acetylene and/or lower olefins, for example, ethylene and propylene, at final cracking temperatures of up to l,500 C. The result of the cracking process depends on the height of the final cracking temperature and the feedstock used.

In the thermal cracking of hydrocarbons, asphalt-like compoundslike soot and coke are formed, which interfere with the quenching and cooling of the cracked gases, particularly if high-boiling feedstock, for example gas oil, is used.

The oil wash serves to free the cracked gases from these constituents before a further treatment and to cool them. The wash oil is generally circulated through the washing apparatus and the said impurities are removed by drawing off a slip stream of oil.-The circulating wash oil can be kept at a desired temperature level by indirect heat exchange with a cooling agent, for example water.

It is known that the heat absorbed by the cooling agent can be utilized, for example, for the production of steam. When using this known process for the recovery of heat, it is generally desired to recover the heat at as high a temperature as possible, for example, in order. to produce steam of as high a working power and in as large an amount aspossible. The effectiveness of this process is limited, however, by the boiling behavior of the wash oil. If the wash oil is kept at a low temperature to avoid losses by vaporization, more heat can be recovered than at higher temperatures of the wash oil but it is only possible in this case to produce steam of a correspondingly low quality. By using higher temperatures of the wash oil, part of the heat is recovered at a higher temperature, while the other part is entrained by the oil vapor developed in a larger amount and can be recovered in the following process stages only at considerably lower temperature.

The recovery of heat from the cycle of liquid wash oil suffers from the further problem that the heat exchangers needed for the heat absorption are easily contaminated in spite of the discharge of a slip stream of oil usually carried through. As a consequence thereof, the heat transfer is in an increasing manner reduced, heat exchangers of large dimensions are required and only short working periods are possible.

It is an object of the invention to avoid the above disadvantages and recover as large an amount of heat as possible of high temperature from cracked gases obtained by the thermal cracking of hydrocarbons-and eliminate the asphalt-like impurities.

The present invention therefore provides a process in which in a first stage of the oil wash, using temperatures within the boiling range of the wash oil and approximately atmospheric pressures, a cycle of high-boiling oil components is maintained and low-boiling oil components which are recycled from a second wash stage in the form of a condensate are exposed to a direct vaporization by the quenched cracked gases, the saturated mixture of oil vapor and cracked gas formed in the first wash stage is then fed to the second wash stage where the predominant part of the oil is condensed by indirect cooling of the mixture, thecondensing oil entraining the impurities originating from the cracking process, the heat of condensation so obtained is fed to a heater, and the cracked gases are then subjected together with the remaining vapors to a known fractionation and other separating processes.

' In the process of the invention, the heatis recovered in the second wash stage from the vapor phase of the oil, that is at elevated temperature. Asphalt-like constituents which are obtained in the second wash stage cannot deposit as this is prevented by the film of condensate. No heat exchangers are needed in the oil cycle of the first wash stage, troubles caused by asphalt-like constituents and other impurities which otherwise frequently occur thus being avoided. It is therefore possible to use a wash oil of higher boiling range than generally used in a cooled wash oil cycle.

The invention will now be described byway of example only with reference to theaccompanying diagrammatic drawing.

The cracking reactor 1 is charged through pipe 2 with a hydrocarbon mixture and at 3 with cracking.

energy as starting materials. The cracking energy may be supplied, as shown in the drawing, bydirect heat exchange, for example with the help of an oxygen flame or a hydrogen plasma, or by indirect heat exchange. The cracked gases leaving the reactor 1 are quenched, if desired, to a temperature below 850 C by a quenching medium injected through pipe 4.

The first wash stage is represented in simplified manner by washing tower 5 and the second wash stage by condenser 6. The boiler 7 traced in dotted lines serves to utilize the waste heat.

In the first wash stage a temperature within the boiling range of the wash oil is maintained under approximately atmospheric pressure. For adjusting the temperature, the amount of quenching medium introduced through pipe 4 and/or the boiling range of the wash oil may be controlled, for example by introduction or removal of wash oil through pipes 14, 15 and 16.

Liquid wash oil is cycled from the lower part of washing tower 5 through pipe 8 via pump 9 to the upper part of the tower.

The cracked gas flows through the first wash stage (washing tower 5) via pipe 10 to the second wash stage (condenser 6) and finally through pipe 11 to a fractionation not shown in the drawing. The cracked gas conducted in this manner carries oil vapor from the first wash stage into the second wash stage. By indirect contact of the mixture of cracked gas and vapor-with a cooling agent, oil condensate is formed. The oil condensa'te is returned through pipes 12 via pump 13 to the first wash stage and again vaporized by the cracked gases.

The heat liberated from the washing agentin the second wash stage is givenoff to boiler 7 where it is utilized for the production of steam. i

In normal operation of the process two cycles of wash oil are maintained. In one cycle only the heavy wash oil components participate. No heat exchangers are used in this cycle. The oil circulating in this cycle wets the walls of washing tower 5 of the first wash stage and keeps them free from incrustations. Washed-out or absorbed impurities are discharged through pipe 14. The second cycle takes place with change of phases and is fed through both wash stages. In the first wash stage, the components of lower boiling range of the wash oil which participate in this cycle are vaporized, while they are condensed in the second wash stage.

1n the second wash stage no further cycle of wash oil is needed because the wash oil which is condensing sufficiently wets the cooling surfaces and washes the impurities originating from the cracking process back into the first wash stage.

By providing the second wash stage with several condensers which are connected in series with respect to the passage of the cracked gases, the oil residues entrained by the gas can be kept small and waste heatof different grades of effectiveness can be obtained. The removed wash oil is replaced by introducing fresh oil and returning oil from other process stages through pipes 16 and/or by the oil fractions obtained in the cracking reaction.

The following Examples serve to illustrate the invention.

EXAMPLE 1 In a thermal cracking process for the manufacture of acetylene 330 kilograms water having a temperature of 80 C were used as quenching agent for 1,000 Nrn of cracked gas having an enthalpy of 0.6 mio kcal (basis at 0 C, gaseous). By quenching to a temperature of 700 C, 1,410 Nm mixture with an enthalpy of 0.43 mio kcal were obtained. This mixture was treated by the process of the invention. The oil wash was carried out with an aromatic oil of an average molecular weight of 240 kg/kmol. In the first stage of the oil wash about 8,000 kg/h oil were recycled, and 3,000 kg/h oil, mainly recycle oil returned from the second wash stage and a proportion of fresh oil, were vaporized. By proceeding in this manner, an equilibrium temperature of 250 C was obtained in the first wash stage. The mixture of gas and vapor leaving the first wash stage was composed of cracked gas plus steam and oil vapor. The cracked gas plus steam had an enthalpy of 0.13 mio kcal, while the oil vapor had an enthalpy of 0.3 mio kcal. The predominant part of the heat of the cracked gas had therefore been used in the first stage for the vaporization of the oil.

1n the second stage the major part of the oil vapor was condensed. The condensate so obtained constituted the washing liquid. lt washed the heat exchanging surfaces and entrained the impurities originating from the cracking process. The mixture of gas and vapor leaving the second wash stage had a temperature of 200 C and contained 600 kilograms of residual oil, 0.105 mio kcal being discharged with the cracked gas plus steam and 0.06 mio kcal with the oil vapor.

Of the amount of heat of 0.325 mio kcal available in the cracked gas 0.265 mio kcal were transferred to the cooling agent in the second wash stage. By cooling with boiling water, steam of a quality of 180 C/ 9 atmospheres gage could be produced. This corresponded to a yield of waste heat of 81.5 percent.

By a conventional single-stage washing process with cooling in an oil cycle a steam quality of 136 C/2.3 atmospheres gage would at best be obtained with a yield of waste heat of 81.5 percent or, with a yield of waste heat of 55 percent, a steam quality of C/ 9 atmospheres gage.

EXAMPLE 2 By a cracking of gas-oil directed to the manufacture of low olefins a cracked gas of an enthalpy of 600 kcal/Nm (basis at 0 C, gaseous) was obtained. The cracked gases were cooled to 320 C by injecting and vaporizing 3.6 kg/Nm oil condensate most 'of-which being returned from the second wash stage, and were then washed at 320 C by a cycle of high-boiling wash oil components. The excess of oil obtained in the cracking reaction was removed through pipes 14 and 15, thus ensuring an effective control of the composition and the temperature of the first cycle of wash oil.

The cracked gas enriched with heavy oil vapors had an enthalpy of 200 kcal/Nm at the outlet of the first wash stage, the predominant part of the heat of the cracked gas, i.e., 400 kcal, being contained in the gas in the form of oil vapor.

In the second wash stage the mixture of cracked gas and oil vapor was cooled to 260 C by indirect cooling whereby 2.7 kg/Nm oil condensate were obtained which served as wash oil and kept the heat exchanging surfaces free from contamination.

By the indirect cooling waste heat in the amount of 250 kcal/Nm was removed which could be used, for example, for the production of saturated steam of 25 atmospheres gage.

In the known processes the heat of the cracked gas is removed by double heat exchange, that is first from the cracked gas to the liquid wash oil and second by indirect heat exchange in the oil cycle 8. The predominant part of the heat of the cracked gas can only be obtained at a considerably lower temperature since the wash oil in the oil cycle must be kept at a lower-temperature to avoid vaporization. The saturated steam so obtained therefore has a pressure of only about 10 atmospheres gage.

What is claimed is:

gases obtained by the thermal cracking of hydrocarbons in the production of acetylene and/or lower olefins, wherein said cracked gases are quenched to stabilize their components and then subjected to an oil wash, said process comprising the steps of, subjecting said cracked gases to a first oil wash stage utilizing an oil having high-boiling temperature oil components, continuously recycling said high-boiling temperature oil components through said first stage without'inte rmediate cooling thereof, maintaining the temperature in said first stage at less than the boiling temperature of said high-boiling temperature oil components, maintaining said first stage under atmospheric pressure, simultaneously subjecting said cracked gases in said oil wash of said first stage to an oil having components of lower-boiling temperatures than those of said high-boiling temperature oil components and less than the temperature maintained in said first stage, whereby in said first stage said lower-boiling temperature oil components vaporize to remove heat from said cracked stage to cool and condense a substantial portion of said lower-boiling temperature oil components and transfer the heat of condensation to said fluid whereby the condensing oil components in said second stage wash said cracked gases and remove impurities therefrom, and

thence, returning the condensed lower-boiling temperature oil components, without further cooling, to

said first stage for use there in said step of subjecting said cracked gases to an oil having components of lower-boiling temperatures than those of said higherboiling temperature oil components.

2. The process as defined in claim 1 including the steps of passing the heated cooling fluid from said con-' denser to a waste heat boiler and passing the cracked gases from said second stage to a fractionator. 

2. The process as defined in claim 1 including the steps of passing the heated cooling fluid from said condenser to a waste heat boiler and passing the cracked gases from said second stage to a fractionator. 